Implement DataInputStream

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix DataOutputStream#writeUTF

This developer did not read the specs closely enough and missed that
the length of the byte array needs to be written out first, so that
DataInputStream#readUTF has a chance of reading the string back.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>